Trigonal molecules diastereotopic atoms

Just as there are enantiotopic and diastereotopic atoms and groups, so we may distinguish enantiotopic and diastereotopic faces in trigonal molecules. Again we have three cases (1) In formaldehyde or acetone (G), attack by an achiral reagent A from either face of the molecule gives rise to the same transition state and product the two faces are thus equivalent. (2) In butanone or acetaldehyde (H), attack by an achiral A at one face gives a transition... 

The concept of heterotopic atoms, groups, and faces can be extended from enantiotopic to diastereotopic types. If each of two nominally equivalent ligands in a molecule is replaced by a test group and the molecules that are generated are diaster-eomeric, then the ligands are diastereotopic. Similarly, if reaction at one face of a trigonal atom generates a molecule diastereomeric with that produced at the alternate face, the faces are diastereotopic. 

If we want to create a new chiral centre in a molecule, our starting material must have prochirality —the ability to become chiral in one simple transformation. The most common prochiral units that give rise to new chiral centres are the trigonal carbon atoms of alkenes and carbonyl groups, which become tetrahedral by addition reactions. In all of the examples you saw in the last section, a prochiral alkene (we can count enolates as alkenes for this purpose) reacted selectively on one face because of the influence of the chiral auxiliary, which made the faces of the alkene diastereotopic. 

Addition to carbonyl groups in chiral molecules is perhaps the best-known example of a reaction involving preferential attack at one of two diastereotopic faces of a trigonal atom, and has been referred to earlier (see Scheme 2.5, entry 5, p. 57). The major diastereomer formed in these processes may be predicted on the basis of... 

Addition to carbonyl groups in chiral molecules is perhaps the best-known example of a reaction involving preferential attack at one of two diastereotopic faces of a trigonal atom, and has been referred to earlier (see Scheme 2.5, entry 5, p. 83). The major diastereomer formed in these processes may be predicted on the basis of an empirical rule proposed by Donald J. Cram Like all empirical rules, Cram s rule is experimentally rather than mechanistically based, and should not be considered as an attempt to explain the observed facts, but to correlate them. If the substrate is oriented so that the two smallest groups of the chiral center flank the carbonyl group, then the major diastereomer produced will correspond to addition of the new ligand from the side of the smallest substituent. 

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